1. Field of the Invention
The present invention relates to an optical-fiber detection device which involves testing for good performance. The invention proposes a device of particularly simple constructional design which is applicable in particular to detection processes involving the use of a number of sensors, the information provided by these sensors being transmitted by one and the same system of optical fibers between the different sensors and a common electronic processing unit.
2. Description of the Prior Art
Optical-fiber detection devices are already known in which optical fibers are employed for transmitting light radiation from a source usually consisting of a laser diode to a sensor and for conveying light information produced at the sensor along a return path to a unit for electronic processing of this information. These devices are constructed in accordance with many alternative designs, especially in the case of the sensor, depending on the nature of the phenomena which it is sought to detect or the quantity which it is desired to measure. Among these devices, however, the invention is primarily concerned with those in which transmission of incident light and return of information from the sensor take place through the same optical fiber and in which the sensor has a sensitive or so-called sensing head designed for operation by reflection of radiation at the end of an optical fiber.
The use of sensors of this type in industrial measurements often entails the need for a reference with which the signal corresponding to the quantity to be measured may be compared. This reference accordingly undergoes the same variations as the measurement signal in the transmission between the sensor and the electronic processing unit. The solutions proposed up to the present time have not yet proved satisfactory. They consist either in placing two different optical fibers on the same path, one for the measuring head which is sensitive to the quantity to be measured, the other for an identical head which is also located within the sensor but isolated from the quantity to be measured, or in transmitting via the same optical fiber radiations having two different wavelengths, only one of which is affected by the quantity to be measured.
The solution proposed by the present invention removes the undesirable effects which may still remain in designs of the prior art as a result of lengths of fibers which may possibly be different and as a result of attenuations of intensity or effects of curvatures of fibers which may be different at two different wavelengths. This invention accordingly proposes to make use of a reference consisting of a signal produced at the level of the sensor at a predetermined time interval with respect to the measurement signal and transmitted by the same optical fiber at the same wavelength. Whether in combination with the aforesaid reference or not, the invention also proposes to design the sensor and the associated electronic processing unit in such a manner as to check continuously whether the sensor is in a normal state of operation.